Because it’s not the role of universities and Computer Science departments to teach a vocation or trade to their (freshmen) students. They’re supposed to teach basic programming concepts and good programming techniques. The best way to do this is starting with a good teaching language. A good teaching language is small and simple and clean, and it gets out of the way of the student so that they can focus their attention on what’s most important, NOT on the peculiarities and quirks of the programming language. See An Open Letter to All Universities.
ECMAScript may have improved a great deal but it is still far from being a good teaching language. And it is still far from being a good programming language in general. Opinions are very divided, of course, but it’s interesting to note that of all the dozens and dozens of tech articles I’ve published over the past three years, this one has become the most read by far…96,000 views and counting! It has really struck a nerve.
I hope other schools are not as misguided as Stanford.